Polyoxymethylene is a preferred material for a wide variety of end-uses. Articles made from polyoxymethylene compositions typically possess extremely desirable physical properties such as high stiffness, high strength and solvent resistance. However, the surfaces of articles made from polyoxymethylene compositions are highly crystalline and exhibit poor adhesion to other materials. It can be very difficult to paint, glue, or print on such surfaces. It is also difficult to overmold such articles with thermoplastic polymers or adhere some other type of layer to the surface. Furthermore, such articles have high surface gloss and light reflected from the surface tends to cause eye irritation. Low surface gloss on fabricated articles on the other hand tends to impart a more aesthetically pleasing high-grade appearance to the articles.
Polyoxymethylene compositions include compositions based on homopolymers of formaldehyde or of cyclic oligomers of formaldehyde. For example trioxane, the terminal groups of which are end-capped by esterification or etherification. The polyoxymethylene compositions also include compositions based on copolymers of formaldehyde or of cyclic oligomers of formaldehyde, with oxyalkylene groups having at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminated or can be end-capped by esterification or etherification. The proportion of the comonomers can be up to 20 weight percent. Compositions based on polyoxymethylene of relatively high molecular weight, for example 20,000 to 100,000, are useful in preparing semi-finished and finished articles by any of the techniques commonly used with thermoplastic materials, such as, for example, compression molding, injection molding, extrusion, blow molding, stamping and thermoforming. It can be desirable to enhance the surface adhesion and reduce gloss in polyoxymethylenes.
Plasticized PVB is an adhesive that can be difficult to handle as a feed to a compounding extruder due to its inherent stickiness. Similarly PVB sheet is a material that can be difficult to work with because of the tendency to adhere to itself. Recently it has been found that PVB can be blended with other materials to obtain composites that have a reduced is tendency to self-adhere. See for example, WO 02/12356 directed to a process for preparing pellets from PVB scrap material. Heretofore it would not have been possible to obtain suitable blends of PVB and polyoxymethylene polymers.
It has been found that polyacetal compositions that include free-flowing PVB do not have the same degree of toughness as the polyacetal prior to inclusion of the PVB. Use of conventional tougheners, while effective in toughening many thermoplastic polymer compositions, can increase the gloss of an article comprising said tougheners. It is an objective of the present invention to produce low-gloss products, and therefore conventional tougheners that increase gloss are not suitable for use herein. For example, polyurethanes are incorporated in U.S. Pat. Nos. 4,640,949; 4,804,716; 4,845,161; 5,286,807 as tougheners, but also increase gloss. U.S. Pat. Nos. 5,258,431 and 5,484,845 describe polyacetal compositions comprising core shell resin.
It is an object of the present invention to provide PVB-enhanced polyoxymethylene (polyacetal) compositions that have enhanced surface adhesion, that are tough, and that have low surface gloss.